1. Field of the Invention
The present invention relates to an apparatus for detecting particulate matter and a correction method of apparatus for detecting particulate matter that is, for example, used in an exhaust gas purification system of a vehicle internal combustion engine. The apparatus for detecting particulate matter detects the particulate matter present in measured gas.
2. Description of the Related Art
In a diesel engine of an automobile and the like, a diesel particulate filter (hereinafter referred to accordingly as “DPF”) is provided on an exhaust gas path. The DPF collects environmental pollutants included in exhaust gas, particularly particulate matter (PM) mainly composed of soot particles and soluble organic fractions (SOF). The DPF is made of a porous ceramic having excellent heat resistance. The DPF captures the PM as a result of the exhaust gas passing through a partition wall having numerous fine pores.
When the amount of collected PM exceeds an allowable amount, the DPF becomes clogged. Negative pressure may increase. Alternatively, the amount of PM escaping through the DPF may increase. Therefore, collection capability is required to be recovered by a regeneration process of the DPF being periodically performed.
In general, the regeneration timing of the DPF is determined by the detection of increase in differential pressure at both ends of the DPF caused by increase in the amount of collected PM. Therefore, a differential pressure sensor is provided that detects the difference in pressure upstream and downstream from the DPF.
The DPF is regenerated by high-temperature exhaust gas being introduced into the DPF through heating using a heater or by post-injection, and the PM being removed by burning.
On the other hand, a PM detection sensor is also proposed that directly detects the PM in the exhaust gas, for example, in JP-A-S59-197847 and JP-A-2008-502892. The PM detection sensor is, for example, provided downstream from the DPF and measures the amount of PM escaping through the DPF. Therefore, the PM detection sensor can be used in an on-board diagnosis (OBD) device to monitor an operating state of the DPF or to detect abnormalities (such as cracks and damage).
Furthermore, use of the PM detection sensor in place of the differential pressure sensor to determine the regeneration timing of the DPF is also being discussed. In this instance, the PM detection sensor is provided upstream from the DPF and measures the amount of PM entering the DPF.
JP-A-S59-197847 discloses an electrical-resistance-type smoke sensor. The smoke sensor is configured such that a pair of comb-shape electrodes is formed on a front surface of a substrate having insulating properties, and a heating element is formed on a back surface of or within the substrate. This type of smoke sensor takes advantage of smoke (particulate carbon) having conductivity, and detects electrical resistance generated when smoke accumulates between the electrodes that serve as a detecting section. Heat-resistant insulating material is used as substrate material. Noble metal such as platinum, silver or the like is used as electrode material. A pair of electrodes is formed by screen-printing noble metal paste on a front surface of a plate-shaped substrate.
On a back surface of the substrate, a heating element is formed at the opposite of the electrode. The detecting section is heated to a desired temperature such as from 400° C. to 1000° C., thereby burning away the deposited smoke. Then, inter-electrode resistance is measured. As a result, detection capability of the sensor is restored.
JP-A-2008-502892 discloses a method for controlling deposition of soot on a sensor. In this method, a high voltage is applied between sensing electrodes. An electric field is generated between the electrodes. The PM passing near the sensing electrodes is attracted by electrostatic attractive force generated by the electric field. Collection of PM is promoted. The collected PM is accumulated between the sensing electrodes. As a result of measurement of the electrical resistance between the sensing electrodes that changes depending on the amount of accumulated PM, the amount of accumulated PM is measured.
The electrical characteristics used to detect the PM within a gas to be measured is not limited to the electrical resistance that changes depending on the amount of PM accumulated between the sensing electrodes. Various electrical characteristics, such as capacitance or changes in current accompanying electrochemical reaction, can also be used.
JP-A-2010-32488 discloses an apparatus for detecting particulate matter that uses resistance, inductance, capacitance, and impedance as the electrical characteristics. The apparatus for detecting particulate matter includes a plate-shaped first electrode, a second electrode, a power supply for dust collection, a pair of measuring electrodes, a characteristic measuring means, and a means for calculating amount of particulate matter. One surface of the first electrode is covered by a dielectric (referred to as an inter-electrode dielectric). The second electrode forms a pair with the first electrode. The power supply for dust collection applies voltage. The pair of measuring electrodes is disposed on the surface of the inter-electrode dielectric such as to oppose each other. The second electrode is disposed on the side of the one surface of the first electrode with a space therebetween. A gas containing PM flows through this space. Electricity is discharged as a result of voltage applied between the first electrode and the second electrode. The characteristic measuring means measures the electrical characteristics between the pair of measuring electrodes. The means for calculating amount of particulate matter determines the amount of PM collected on the surface of the inter-electrode dielectric, based on the amount of change in electrical characteristics measured by the characteristic measuring means.
On the other hand, JP-A-2010-54432 discloses a sensor detecting an amount of carbon. The amount of carbon detection sensor includes at least a proton conductor, an electrode pair, and a power supply. The proton conductor is composed of a solid electrolyte having proton conductivity. The electrode pair is composed of a measuring electrode and a reference electrode formed on the surface of the proton conductor. The power supply applies a predetermined current or voltage between the electrode pair. The measuring electrode is disposed on the opposite side of the gas to be measured and the reference electrode is isolated from the gas to be measured. As a result of measurement of the changes in current or voltage flowing by electrochemical reaction with the PM within the gas to be measured on the surface of the measuring electrode, the amount of PM is detected.
In general, sensors, such as PM detection sensors and oxygen detection sensors, are fixed to a flow path of the gas to be measured, via a housing. For example, in JP-A-2009-97868, a detecting element placed within the gas to be measured is protected by being covered by a substantially cylindrical cover body having a predetermined hole.
However, in an actual manufacturing process, it is difficult to match the direction of the detecting element of the sensor element with the direction of the inlet for gas to be measured provided in each cover body when assembling the sensor element and the cover body to the housing, for each sensor. When the direction of the sensor element is matched with the direction of the cover body for each sensor, work efficiency becomes extremely poor. Manufacturing costs of the particulate matter detection sensor increases.
On the other hand, when the particulate matter detection sensor is assembled with no regard for the direction of the sensor element and the direction of the cover body, work efficiency improves. However, the direction of the detecting element and the direction of the inlet for gas to be measured provided in the cover body vary. The flow of gas to be measured that is introduced into the cover body differs with each sensor. As a result, individual differences in output in relation to the collecting performance of particulate matter contained within the gas to be measured in the detecting element and the amount of accumulated particulate matter become greater. Reliability as a sensor significantly decreases.
In some instances, a plurality of electrodes for collecting may be disposed within a single sensor element. The collection electrode collects the PM using electrostatic attractive force by applying an electric field to the PM to be detected. In addition to the collection electrodes, a sensing electrode for detecting electrical characteristics may be provided. In these instances, variations inevitably occur in the actual distance between electrodes. The variations in inter-electrode distance cause variations in field strength generated between the electrodes. The amount of PM collected in the detecting element changes, and individual differences in detection results occur.
The distance between electrodes that have been actually manufactured can be measured by image processing or the like. However, it is very difficult to reduce the variations in output results through classification of each sensor element into ranks based on the distribution of inter-electrode distance within a manufacturing lot or the like. Cost effectiveness is also poor.
Moreover, a method is also known in which the sensor element is provided with a through hole. The electrodes for collecting are set above and below the through hole. As a result of an electric field being generated between the collection electrodes, the PM is collected. However, even in this method, ensuring the distance between electrodes is difficult. Individual differences occur in the field strength that is actually generated. Variations in output results occur.